Stable isotope composition (δ13C and δ15N values) of slime molds: placing bacterivorous soil protozoans in the food web context

Rationale Data on the bulk stable isotope composition of soil bacteria and bacterivorous soil animals are required to estimate the nutrient and energy fluxes via bacterial channels within detrital food webs. We measured the isotopic composition of slime molds (Myxogastria, Amoebozoa), a group of soil protozoans forming macroscopic spore-bearing fruiting bodies. An analysis of largely bacterivorous slime molds can provide information on the bulk stable isotope composition of soil bacteria. Methods Fruiting bodies of slime molds were collected in a monsoon tropical forest of Cat Tien National Park, Vietnam, and analyzed by continuous-flow isotope ratio mass spectrometry. Prior to stable isotope analysis, carbonates were removed from a subset of samples by acidification. To estimate the trophic position of slime molds, their δ13C and δ15N values were compared with those of plant debris, soil, microbial destructors (litter-decomposing, humus-decomposing, and ectomycorrhizal fungi) and members of higher trophic levels (oribatid mites, termites, predatory macroinvertebrates). Results Eight species of slime molds represented by at least three independent samples were 3–6‰ enriched in 13C and 15N relative to plant litter. A small but significant difference in the δ13C and δ15N values suggests that different species of myxomycetes can differ in feeding behavior. The slime molds were enriched in 15N compared with litter-decomposing fungi, and depleted in 15N compared with mycorrhizal or humus-decomposing fungi. Slime mold sporocarps and plasmodia largely overlapped with oribatid mites in the isotopic bi-plot, but were depleted in 15N compared with predatory invertebrates and humiphagous termites. Conclusions A comparison with reference groups of soil organisms suggests strong trophic links of slime molds to saprotrophic microorganisms which decompose plant litter, but not to humus-decomposing microorganisms or to mycorrhizal fungi. Under the assumption that slime molds are primarily feeding on bacteria, the isotopic similarity of slime molds and mycophagous soil animals indicates that saprotrophic soil bacteria and fungi are similar in bulk isotopic composition. Copyright © 2015 John Wiley & Sons, Ltd.